This invention relates to a process of producing composite powder which consists of particles of a metallic sulfide, carbon, activated charcoal, metallic carbide, metallic oxide or the like coated with a noble metal. As used throughout this description, the term "noble metal" is intended to embrace gold, silver, and platinum group metals.
Articles made by blending a noble metal powder and a powder of a metallic sulfide such as molybdenum disulfide or tungsten disulfide, carbon powder of amorphous carbon or graphite, powder of a metallic carbide such as tungsten carbide, titanium carbide, or silicon carbide, or powder of a metallic oxide such as tin oxide or silver oxide and then compacting and sintering it to shape have found extensive applications as, for example, electric contacts, metallic brushes, sliding contactors, and oilless bearings. Also in practice are mixing such a blended powder with a synthetic resin to a paste form and applying the paste on an insulation substrate to produce a coated film layer of low resistance. Further, activated charcoal coated with silver is in use for ozone reduction.
Mere mixing of such a powder of a metallic sulfide, carbon, activated charcoal, metallic carbide, or metallic oxide with a noble metal powder will not give a uniformly dispersed system, and when the mixture is compacted and sintered to be a resistor, for example, the product will show wide irregularity in resistivity. In another application as a sliding contactor, it will fail to provide adequate slidability or contact resistance.
Therefore, in an effort to obtain sintered bodies of thoroughly dispersed powder mixtures, it has been proposed to form composite powder by coating the individual particles of one of the aforementioned powders with a noble metal by chemical reduction plating and then to sinter a green compact of the composite powder. The composite powder gives a sintered body of good quality, because of improved uniformity in dispersion and added strength on sintering. For example, when carbon powder and gold or siver powder are blended with synthetic resin, the employment of the composite powder of carbon and gold or silver will improve the homogeneity of the mixed system, and the compounding ratio of the carbon and gold or silver can be closely controlled by regulating the thickness of gold or silver coating.
In chemical reduction plating of the aforesaid powders with noble metals, the methods thus far proposed have required some pretreatment, such as a sensitizing or activation treatment. In addition, the plating solution is very expensive. A more recent proposal, made in Japanese Patent Application Public Disclosure No. 39403/1977, pertains to silver coating of carbonaceous powder. According to the method, the powder of a carbonaceous substance is placed in a silver nitrate solution to occlude the latter, an organic acid salt is added to the silver nitrate solution so as to convert silver nitrate to an organic acid salt of silver, the resultant is filtered to obtain a carbonaceous substance that has occluded the organic acid salt of silver, the carbonaceous substance is thrown into pure water, and then a reducing agent such as a hydrazine solution is added to it thereby to reduce the occluded organic acid salt of silver. The method has drawbacks, however. The amount of silver coating varies with the surface area to be plated of the carbonaceous particles, and is especially influenced by the existence of micropores. The salt used will be partly left behind in the micropores, adversely affecting the quality of the product in subsequent steps of processing. The chemicals to be employed are expensive and call for very complicated processing procedure.
In view of these, there has been a great need in the art for the development of a method for coating a powder of a metal sulfide, carbon, activated charcoal, metallic carbide, or metallic oxide with a noble metal through a simplified process without the necessity of a special pretreatment or an expensive chemical agent.
We have already proposed in our copending Japanese Patent Application No. 134726/1977 a process for coating metal sulfide particles with copper by a cementation reaction. The process was directed only to the combinations of metal sulfides and copper. After further investigations it has now been confirmed that such cementation technique is also applicable to coating of the above-mentioned powders with noble metals.